State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, Nanjing 211816, Jiangsu, P. R. China.
School of Energy and Environment, Southeast University, Nanjing 210096, Jiangsu, P. R. China.
J Am Chem Soc. 2023 May 24;145(20):11431-11439. doi: 10.1021/jacs.3c03198. Epub 2023 May 10.
Covalent organic frameworks (COFs) have showcased great potential in diverse applications such as separation and catalysis, where mass transfer confined in their pore channels plays a significant role. However, anisotropic orientation usually occurs in polycrystalline COFs, and perpendicular alignment of COF pore channels is ultimately desired to maximize their performance. Herein, we demonstrate a strategy, solvent vapor annealing, to reorient COF pore channels from anisotropic orientation to perpendicular alignment. COF thin films are first synthesized to have flexible N-H bonds in their skeletons, thus having structural mobility to enable molecular rearrangement. A solvent with low relative permittivity and a conjugated structure is then identified to have a strong affinity toward the COFs, allowing its vapor to easily penetrate into the COF interlayers. The solvent vapor weakens the π-π interaction and consequently allows the COF monolayers to dissociate. The COF monolayers undergo a reorientation process that converts from random stacking into the face-on stacking fashion, in which the through COF pores are perpendicularly aligned. The aligned COF film exhibits high separation precision toward ions featuring a size difference down to 2 Å, which is 8 times higher than that of the anisotropically oriented counterpart. This work opens up an avenue for COF orientation regulation by solvent vapor annealing and reveals the essential role of the perpendicular alignment of COF pore channels to enable precision separations.
共价有机框架（COFs）在分离和催化等多种应用中展现出巨大的潜力，其中质量传递在其孔道中的受限发挥了重要作用。然而，多晶 COFs 通常会出现各向异性取向，而 COF 孔道的垂直排列最终是为了最大限度地提高其性能。在此，我们提出了一种策略，即溶剂蒸气退火，以将 COF 孔道从各向异性取向重新定向到垂直排列。首先合成 COF 薄膜，使其骨架具有灵活的 N-H 键，从而具有结构迁移性以实现分子重排。然后确定一种具有低介电常数和共轭结构的溶剂，使其对 COF 具有很强的亲和力，从而允许其蒸气容易地渗透到 COF 层间。溶剂蒸气削弱了 π-π 相互作用，从而使 COF 单层解离。COF 单层经历一个重定向过程，从随机堆积转变为面向上的堆积方式，其中贯穿 COF 的孔垂直排列。定向 COF 薄膜对具有 2Å 大小差异的离子表现出高分离精度，比各向异性取向的 COF 高 8 倍。这项工作通过溶剂蒸气退火为 COF 取向调控开辟了一条途径，并揭示了 COF 孔道垂直排列对于实现精确分离的重要作用。
